Method of testing plating solutions



Patented Mar. 22, 1949 METHOD OF TESTING PLATING SOLUTIONS Joseph S. Buser, Marland Heights, W. Va., assignor to National Steel Corporation, a corporation of Delaware No Drawing. Application February 12, 1943, Serial No. 475,695

Claims. 1

The present invention relates to testing methods for electrolytic tin plating baths. More particularly, the invention relates to a convenient and ready method for determining the molecular ratio of fluoride to tin in a tin-fluoride plating bath.

Recently developed electroplating baths for electroplating of tin on base metal include tin salts such as stannous sulphate or stannous chloride dissolved in water with the addition of an alkali fluoride such as sodium fluoride or potassium fluoride. Sometimes the double form of fluoride salt is used either to get more fluoride into the bath in proportion to the sodium or potassium or where the pH of the solution is more conveniently controlled in this manner. Sometimes the pH is controlled by the addition of an acid such as hydrochloric or hydrofluoric acid. In this type bath the acid content is maintained low, preferably within the pH range of between 2 and 5. Common addition agents are used where necessary as indicated by Hull cell tests and practical operations.

The stannous sulfate or stannous chloride is used to create a reservoir of stannous tin ions in the solution. The alkali fluoride appears to have two functions. Thefluoride ion stabilizes the stannous tin ion and prevents hydrolysis of the stannous tin ion to cause it to change to the hydrate which precipitates out of solution. Additionally, the fluoride ion forms a stannous fluoride complex which gives superior plating effects.

Under certain conditions there is an objectionable amount of sludging or precipitate formed in this type bath. During plating especially, turbulence of the bath results in aeration and the resulting oxidation of the solution causes a precipitate to form. This precipitate is believed to be an insoluble complex formed by the reaction of stannic ions and the fluoride. The stannic ions are the result of oxidation of the stannous ions in the solution. The complex in precipitating removes from the solution more fluoride than tin and as a result the bath becomes fluoride poor.

I. have discovered that under certain conditions the amount of this form of precipitate is inversely related to the molecular ratio of fluoride to tin in the bath and from this phenomenon I have developed a convenient and ready method of testing a fluoride-tin plating solution.

An important object of the present invention is to provide a ready and convenient method of testing fluoride-tin plating solutions to determine the fluoride tin ratio.

Further objects and advantages of the invention will become apparent from. a study of the following specification.

When a plating bath has been in use for some time and due to dragout of solution by the material being plated the solution needs replenishing, a portion of the bath is removed in present practice and replenishing components added thereto in a mixing tank. Due to some precipi tation besides the dragout, the fluoride to tin ratio in the bath has usually changed from the initial value. In prior practice tin salts and fluoride salts were added to the bath in about the same proportions as the initial makeup. It will be apparent that in such case with the bath being fluoride poor at the time of replenishing it will remain fluoride poor. For a reason at present unknown to me the fact that the fluoride to tin ratio of a bath is low results in greater precipitation. Thus, the action is cumulative since with each increment of precipitation the bath becomes more fluoride poor and there is still more precipitation.

In utilizing the invention I add suflicient tin to the solution in the mixing tank to bring the tin up to the desired level; then I add the fluoride salts until the test covered by the present application shows that there is a sufilcient amount in relation to the tin present.

The test of the present invention is based on the phenomenon that in a solution containing a tin salt and a fluoride salt the addition of an oxidizing agent results in a precipitate, the amount of which is inversely related to the ratio of fluoride present to tin present. I have found that with certain baths having a fluoride to tin molecular ratio of 6:1 at a pH of 2.5, substantially no precipitate is formed. Obviously such a condition would represent the optimum ratio for Standard No. 1Molecular ratio fluoride to tin 2:1

Fifty-seven grams of commercial tin chloride SnCl2.2H2O were mixed with 23 grams of com- Standard N o. 2-Molecular ratio fluoride to tin 3 :1

The same procedure was followed as outlined above with regard to standard No. 1 except that the amount of sodium fluoride was increased to 35 grams to give the desired molecular ratio. The amount of precipitate resulting was 5 cc.

Standard No. 3Molecular ratio fluoride to tin 4:1

The same procedure as in the preceding standards except that the sodium fluoride was increased to 47 grams. The precipitate resulting was .9 cc.

Standard No. 4Molecular ratio fluoride to tin 5 :1

For this standard the amount of fluoride was increased to 59 grams. The precipitate in the test tube was 0.2 cc.

Standard No. Molecular ratio fluoride t0 tin 6 :1

The amount of sodium fluoride added to the bath was 70 grams. There was substantially no precipitate in the test tubes.

In using the above standards while replenishing a plating solution to the named tin content the tin salt is added to the solution to be replenished in the mixing tank until the desired tin content is reached. Sufiicient fluoride may be added at the same time to prevent hydrolysis. Then by taking 10 cc. of the resulting solution, adding 1 cc. of 3% hydrogen peroxide, shaking the solution for seconds and allowing 15 minutes to settle, the amount of precipitate in the test tube when compared with the five standards will show the approximate molecular ratio of fluoride to tin in the bath. A rapid calculation will indicate how much more fluoride should be added and if desired a final test with another 10 cc. of the resulting solution will show whether or not the desired molecular ratio of fluoride to tin has been reached.

I have found that a bath in which the tin is supplied in the form of stannous sulphate gives about the same results; thus where the tin content is the same as in the solution referred to above, standards can be prepared in the same manner using the amount of stannous sulphate necessary to give this tin content and resulting in about the same amount of precipitate for each standard.

It will be apparent that other less common tin bearing salts could be used so long as the proper weights are calculated for giving the desired tin content to the solution. Thus, any tin plating bath wherein fluoride is an important constituent may be tested in this manner.

I have found that change in the pH upwardly slightly increases the amount of precipitate for each standard. Changing the pH downwardly results in a decrease in the precipitate until a pH in the neighborhood of 1.7 or 1.8 is reached at which point, with the tin content referred to specifically above, precipitate tends to disappear.

It is to be understood that with difierent tin concentrations the amount of fluoride which can be gotten into the bath before saturation is reached will vary. In the solutions specifically referred to above the molecular ratio of fluoride to tin of 6:1 closely approaches the saturation point for the fluoride salt.

To those skilled in the art it will be obvious that in preparing the standards any agent which Will change a tin salt from the bivalent to quadrivalent state will produce substantially the same results. The most common of these reagents would be: chlorine, th hypochlorites, the -chlo-' rates, iodine, the iodates, the arsenites, the arsenates, and the peroxides of sodium and potassium. The term oxidizing agent as used in this specification and the appended claims refers to any substance described herein or any other substance which will change stannous ions in solutions to stannic ions.

I claim:

1. A method of testing tin-fluoride tin plating solutions comprising setting up standards by determining the amount of precipitate formed when a known quantity of oxidizing agent is added to each of a plurality of test solutions of the same known quantity having predetermined different molar ratios of fluoride to tin, taking a sample of the same quantity of the plating solution to be tested, and adding the same quantity of oxidizing agent under the same conditions to the sample whereby the amount of precipitate formed in the sample may be compared with the amount of precipitate formed with the standards so as to determine the molar ratio of fluoride to tin in the solution.

2. A method of testing tin-fluoride tin plating solutions comprising setting up standards by de-' termining the amount of precipitate formed when a known quantity of hydrogen peroxide is added to each of a plurality of tin-fluoride solutions of the same known quantity having known different molar ratios of fluoride to tin, taking a sample of the same quantity of the plating solution to be tested, and adding the same quantity of hydrogenperoxide to the sample under the same conditions whereby the amount of precipitate formed in the sample may be compared with the amount of precipitate formed with the standards to determine the molar ratio of fluoride to tin in the plating solution being tested.

8. A method of testing tin-fluoride tin plating solutions comprising setting up standards by determining the amount of precipitate formed.

when a known quantity of oxidizing agent is added to each of a plurality of test solutions of the same known quantity having predetermined different molar ratios of fluoride to tin, taking a sample of the same quantity of the plating solution to be tested, adding the same quantity of oxidizing agent to the sample under the same conditions, and juxtaposing the sample and at least one of the standards for comparison of the amount of precipitate formed in the sample with the amount formed in the juxtaposed standard whereby the molar ratio of fluoride to tin in the plating solution being tested can be determined.

4. A method of testing tin-fluoride tin plating solutions comprising setting up standards by determining the amount of precipitate formed when a known quantity of hydrogen peroxide is added to each of a plurality of test solutionsof the same known quantity having predetermined different molar ratios of fluoride to tin, taking a sample of the same quantity of the plating solution to be tested, adding the same quantity of hydrogen peroxide to the sample under the same conditions, and juxtaposing the sample and at" least one of the standards for comparison of the amount of precipitate formed in the sample with the amount formed in the juxtaposed standard whereby the molar ratio of fluoride to tin in the plating solution being tested can be readily determined.

5. A method of testing tin-fluoride tin plating solutions comprising setting up standards by determining the amount of precipitate for-med when a known quantity of oxidizing agent is added to each of a plurality of testing solutions of the same known quantity having predetermined different molar ratios of fluoride to tin, taking a sample of the same quantity of the plating solution to be tested, adding the same quantity of oxidizing agent to the sample under the same conditions whereby the amount of precipitate formed in the sample may be compared with the amount of precipitate formed with the standards so as to determine the molar ratio of fluoride to tin in the plating solution, and adding fluoride to the plating solution in an amount in accordance with the amount of precipitate formed in the sample.

6. A method of testing tin-fluoride tin plating solutions comprising setting up standards by determining the amount of precipitate formed when a known quantity of hydrogen peroxide is added to each of a plurality of testing solutions of the same known quantity having predetermined different molar ratios of fluoride to tin, taking a sample of the same quantity of the plating solution to be tested, adding the same quantity of hydrogen peroxide to the sample under the same condition whereby the amount of precipitate formed in the sample may be compared with the amount of precipitate formed with the standards so as to determine the molar ratio of fluoride to tin in the plating solution, and adding fluoride to the plating solution in an amount in accordance with the amount of precipitate formed in the sample.

7. A method of testing tin-fluoride tin plating solutions comprising setting up standards by determining the amount of precipitate formed when a known quantity of oxidizing agent is added to each of a plurality of test solutions of the same known quantity having predetermined different molar ratios of fluoride to tin, taking a sample of known quantity of the plating solution to be tested, and adding the same relative quantity of oxidizing agent under the same conditions to the sample whereby the amount of precipitate formed in the sample may be compared with the amount of precipitate formed with the standards so as to determine the molar ratio of fluoride to tin in the solution.

8. A method of testing tin-fluoride tin plating solutions comprising setting up standards by determining the amount of precipitate formed when a known quantity of hydrogen peroxide is added to each of a plurality of tin-fluoride solutions of the same known quantity having known different molar ratios of fluoride to tin, taking a sample of known quantity of the plating solution to be tested, and adding the same relative quantity of hydrogen peroxide to the sample under the same conditions whereby the amount of precipitate formed in the sample may be compared with the amount of precipitate formed with the standards to determine the molar ratio of fluoride to tin in the plating solution being tested.

9. A method of testing and replenishing tinfluoride tinplating solutions to maintain a fluoride to tin molecular ratio of at least 6:1 comprising replenishing such a solution with tin, adding fluoride to the tin replenishing solution, testing the fluoride replenished solution by taking a sample of known quantity of the solution and mixing a known quantity of oxidizing agent with the sample at a pH value of 2.5 whereby a precipitate is formed when the ratio of fluoride to tin is below 6:1, and adding more fluoride to the solution only when said testing of the fluoride replenished solution produces a precipitate, until a final identical testing of the solution produces no precipitate.

10. A method of testing and replenishing tinfluoride tinplating solutions to maintain a fluoride to tin molecular ratio of at least 6:1 comprising replenishing such a solution with tin, adding fluoride to the tin replenished solution, testing the fluoride replenished solution by taking a sample of known quantity of the solution. and mixing a known quantity of hydrogen peroxide with the sample at a pH value of 2.5 whereby a precipitate is formed when the ratio of fluoride to tin is below 6:1, and adding more fluoride to the solution only when said testing of the fluoride replenished solution produces a precipitate, until a final identical testing of the solution produces no precipitate.

JOSEPH S. BUSER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,542,979 Yant et a1 June 23, 1925 1,587,782 McNeil June 8, 1926 

